1. Field of the Invention
This invention relates to a device for detecting abnormality of the fuel supply system of internal combustion engines, and more particularly to a device of this kind which detects abnormality of the fuel supply system by the use of an air-fuel ratio correction coefficient set in response to an output from an exhaust gas component concentration sensor arranged in the exhaust system of the engine.
2. Prior Art
Conventionally, there has been proposed, e.g. by the present assignee in Japanese Laid-Open Patent Publication (Kokai) No. 6-42382, a technique of detecting abnormality of the fuel supply system of an internal combustion engine, such as deviation of the fuel supply amount from a controllable range due to clogging of a fuel injection valve, biting of foreign matter, aging of the system, etc. According to this technique, an air-fuel ratio correction coefficient is calculated based on an output from an exhaust gas component concentration sensor arranged in the exhaust system of the engine, and it is determined that the fuel supply system is abnormal when an average value of the air-fuel ratio correction coefficient deviates from a predetermined range.
The present assignee has also proposed another technique of detecting abnormality of the fuel supply system by Japanese Patent-Application No. 6-73909. According to this technique, when an average value of the air-fuel ratio correction coefficient deviates from a predetermined range, it is once determined that the fuel supply system is abnormal, similarly to the first-mentioned technique, but if the average value is thereafter changing toward the center of the predetermined range, the calculation of the average value is continued, and when the average value comes to stay within the predetermined range, it is quickly determined that the fuel supply system is normal.
In this abnormality determination, the air-fuel ratio correction coefficient calculated based on the output from the exhaust gas component concentration sensor is readily influenced by purging of evaporative fuel generated from a fuel tank, and when a large amount of evaporative fuel is purged, the air-fuel ratio correction coefficient is controlled to its limit value or a value in the vicinity thereof. As a result, the average value as an abnormality-determining parameter calculated from such extreme values of the air-fuel ratio correction coefficient under the influence of purging of the evaporative fuel is updated toward the limit value as well, which can lead to an erroneous determination of abnormality of the fuel supply system.
To avoid this inconvenience, according to the proposed technique, when the abnormality-determining parameter is updated to a value lower than a predetermined value, the purging of evaporative fuel is once forced to stop and then the fuel supply system is again checked for a fault. If the value of the abnormality-determining parameter increases during the checking to reveal that the above updating of the parameter is due to purging of evaporative fuel, the checking of the system for a fault is inhibited all the time during the present operation of the engine is stopped, so as to prevent degradation of exhaust emission characteristics and driveability of the engine, which would otherwise result from repeated execution/inhibition of purging of evaporative fuel.
However, according to the prior art technique, execution of the checking of the system for a fault is inhibited during the present operation of the engine as mentioned above, which raises a new problem of incapability of detecting an actual fault of the fuel supply system occurring during inhibition of the checking of the system for a fault. Therefore, there has been an urgent demand for a technique or device which eliminates this inconvenience.